Studies to determine the gross responses and morphological and cytological changes induced by CMU, representative of the substituted urea herbicides, were made on barley, onion, and certain dicotyledonous crop plants. Moore barley and Manchu soybean were treated with aqueous suspensions of CMU applied to the soil at 4 rates and at 5 stages of plant development for barley and at 3 stages for soybean. CMU was introduced into Hoagland''s nutrient solution at concentration of 0-3 ppm to determine effects on barley seedlings. CMU-lanolin mixtures, containing 10 mg of the herbicide were applied to various internodes of tomato in greenhouse tests. Roots of sprouting onion bulbs were exposed to aqueous suspensions of CMU at concns. between 0 and 0.5%. Samples of plant organs were removed at several intervals after treatment for morph. and cytological study. Gross responses of barley to CMU toxicity involved loss of turgor, chlorosis, and progressive die-back of older leaves. Mitosis appeared retarded in apical meristems of the shoot, and shoot reduction was accompanied by root reduction both in length and in dry weight. The toxic response appeared first in the epidermis of barley leaves at the margins, then progressed to the mesophyll. Mature vascular tissue was relatively undamaged, although "plugging" of xylem vessels was common. Malformations included tubular leaves, incomplete heading, and abnormally twisted flag leaves. Barley was found to be highly susceptible to CMU at early jointing and at booting as measured by the formative effects of the chemical. Responses included stunting and retardation in spike development and emergence. Plants treated at early jointing gave rise to spikes with only a few mature grains because of the action of CMU on florets at the tip of the spike. Soybean was very susceptible to pre-emergence treatment with CMU at rates above 1/2 lb. per acre. In soybean and tomato treated postemergence with the herbicide, there was chlorosis and progressive development of necrotic areas in the leaves as well as reduction in growth. Acropetal translocation of CMU to the leaves of tomato has been demonstrated. In both soybean and tomato treated with CMU, epidermal cells of the leaf collapsed first, then palisade was affected, and finally the remainder of the mesophyll. In the stem, there was less cambial activity, phloem disorganization, and collapse of interfascicular cambium. Onion root tips from bulbs treated with CMU showed a lack of normal differentiation of the meristem, nuclear breakdown, and disruption of epidermal cells. Mitotic acitivity in roots appeared reduced, but aberrant chromosomes were not observed in Feulgen squashes prepared from the tips. The retardation of growth and maturation in plants treated with CMU as shown in apical meristems and in cambial activity indicates the involvement of fundamental growth processes, the rates and nature of which are altered by CMU. The primary action appears to involve enzyme systems.